The current trend in the design of wind turbines is to increase the length of the blades in order to increase the annual power produced, by means of a compromise between lightness and rigidity. In turn, it is advantageous to have a single platform (“platform” should be understood to be the structural elements of the wind turbine; that is, the tower, the hub and the nacelle) of a single size appropriate for use with blades of different sizes, for different types of location, thus opening new markets to the product, optimising costs.
One of the greatest problems related to increasing the length of the blades while limiting their weight is the increase in flexibility of the same. Said flexibility in combination with an increase in wind speed causes an increase in the deflection of the blades in the direction of the wind. This means that for a windward-facing wind turbine (normal configuration), the tips of the blades are deflected towards the tower.
There exist a number of solutions to prevent the deflected blades from striking the tower, as should this occur, irreparable damage would be caused to both components. A common practice consists of designing the rotor shaft in such a way that it is not horizontal, but is set at an angle from the horizontal, to separate the tips of the blades from the tower when they pass in front of said tower while rotating. This angle is known as the “tilt angle”. The greater said angle, the more flexible, and therefore lighter, the blades may be, having a positive impact on the weight of the remainder of the components and finally on the cost of the whole. However, the effective surface of the rotor is reduced.
Another possibility consists of the use of hubs with a greater coning angle. However, once again, this brings about a reduction in the area swept by the rotor perpendicular to the wind. These known possibilities imply a reduction in the annual energy production.
The wind regime where the possibility of impact of the tip coincides with the corner of the machine's power curve (8-13 m/s) where the thrust is greatest. In this regime, the blade's pitch angle is usually the one which achieves the greatest production possible, commonly called “fine pitch”.
In wind turbines where the aim is to maximise production without any problem of impact between the tip of the blade and the tower, the thrust increases with the wind from its minimum value for connection until the wind turbine reaches its nominal power, the thrust reaching its maximum value in this regime and at blade pitch angles of approximately 0° (fine pitch). Once nominal power has been reached, the blade pitch angle is moved towards a featherd position to reduce energy capture and to maintain power at its nominal value, once again reducing the thrust.
As thrust is the main cause of blade flexion, it may be inferred that the deflection will be greatest close the wind regime of maximum thrust and at blade pitch angles close to fine pitch.
Methods for the collective control of blade pitch angle (to the same extent in all blades) to prevent the collision of the blade with the tower are known in the state of the art. To do this, in problematic wind regimes, i.e. the corner of the wind turbine's power curve (8-13 m/s), the pitch angle of the three blades is increased to avoid that, due to wind gusts, the blades flex excessively.
The greatest drawback of this control method lies in that it moves the three blades to positions which are not optimal from the aerodynamic point of view, resulting in a great loss of annual power.
In an attempt to solve this energy loss, strategies have been developed in the state of the art whereby the blade pitch angle is modified cyclically for each blade exclusively in a sector of rotation where the blade passes in front of the tower (azimuthal angles comprised within 90° and 270°).
This type of control corresponds to that revealed in document US2013045098, wherein a control method is described whose aim is to increase the distance between the tip of the blade and the tower. By means of this method, the blade pitch angle of the blade approaching the tower is controlled, moving it to an angle of lower power capture (i.e. the blade is feathered) when its azimuthal position is detected and it is close to the position where it passes by the tower.
The method described in said document also compensates for the loss of energy associated with the movement toward positive pitch angles of the blade approaching the tower, by turning the other two blades to face the wind more directly. This also enables a reduction in the associated power ripple. The problem is that this only works in nominal power regimes (in other regimes the pitch angle is already optimal, understood to be that which provides the maximum power capture) where there is surplus power.
Given that the minimum distance between the tip of the blade and the surface of the tower occurs when it passes at 180° from its azimuthal position, an additional blade pitch angle term is added, whose value depends on the azimuthal position of the blade and whose maximum value is reached when the blade passes the tower (in an azimuthal position corresponding to approximately 180°, the origin of the reference being the upwardly pointing vertical position of the blade).